Liquid suspension of cerium oxide particles and method for preparing the same

LIQUID SUSPENSION AND POWDER OF CERIUM OXIDE PARTICLES, METHODS FOR THE PREPARATION OF THE SAME AND USE IN POLISHING

The present invention relates to a liquid suspension and a powder of cerium oxide particles, processes for the preparation of the latter and their use in polishing in particular.

The development of the electronics industry requires the increasing use of compositions for polishing various parts such as discs or dielectric compounds. These compositions are in the form of suspensions and they must meet a certain number of characteristics. For example, they must offer a high material removal rate which reflects their abrasive capacity. They must also exhibit the lowest possible defectivity, by defectivity in particular means the rate of scratches exhibited by the substrate once treated with the composition.

For reasons of stability and ease of use, these suspensions must consist of particles of submicron size, ie generally less than 300 nm. In addition, the presence of too fine particles in these suspensions reduces their abrasive capacities. On the other hand, particles that are too large can contribute to an increase in defectivity. There is therefore a need for suspensions in which the particles are monodisperse. It should also be noted that to obtain optimum performance this monodispersity must apply both to the primary particles and to the secondary particles, that is to say the aggregates formed by the primary particles.

It is therefore understood that the development of these suspensions is a complex problem.

The object of the invention is to provide suspensions meeting the conditions described above, that is to say suspensions in which the particles are monodisperse.

For this purpose, the suspension of the invention is a suspension of cerium oxide particles in a liquid phase, and it is characterized in that these particles (secondary particles) have an average size of at most 200 nm, these particles secondary particles consisting of primary particles whose sizes have an average value of at most 100 nm with a standard deviation of at most 20%.

Other characteristics, details and advantages of the invention will emerge even more fully on reading the description which follows, the various concrete but non-limiting examples intended to illustrate it, as well as the appended drawing in which:

- Figure 1 is a photograph obtained by electronic transmission microscopy of a product according to the invention;

- Figure 2 is a photograph obtained by electron transmission microscopy of another product according to the invention.

For the remainder of the description, the expression “suspension of cerium oxide particles” denotes a system consisting of fine solid particles of submicron size based on this oxide, dispersed stably in a liquid phase, said particles possibly possibly , in addition, contain residual amounts of bound or adsorbed ions such as, for example, nitrates or ammoniums.

Still for the remainder of the description, the term “specific surface area” is understood to mean the BET specific surface area determined by nitrogen adsorption in accordance with standard ASTM D 3663-78 established from the BRUNAUER EMMETT- TELLER method described in the periodical The Journal of the American Chemical Society, 60, 309 (1938).

The particles of the suspension are based on cerium oxide which is generally crystalline ceric oxide.

The particles which constitute the suspension of the invention and which have an average size of at most 200 nm are referred to in the remainder of the description as “secondary particles”. These particles are aggregates of other finer particles, aggregated, hereinafter called "primary particles".

According to an important characteristic of the invention, these primary particles are fine and monodisperse. They indeed have an average size of at most 100 nm with a standard deviation of the sizes of at most 20%.

The average value of the size of the primary particles is determined by the technique of X-ray diffraction (XRD). The value measured in DRX corresponds to the size of the coherent domain calculated from the width of the two most intense diffraction lines and using the Scherrer model. This value can also be determined by measuring the BET area.

The standard deviation mentioned in the present invention has the usual mathematical meaning, it is the square root of the variance and it is expressed by the formula:

n being the number of particles taken into account in the measurement,

Xi being the size of a particle i ï being the mean value of the particle size (1 / n £ jx,)

The size of the n different particles is measured from a photograph obtained by transmission electron microscopy (TEM).

This standard deviation can be more particularly at most 15% and even more particularly at most 10%.

The primary particles can more particularly have sizes which have an average value of at most 80 nm, even more particularly of at most 60 nm.

These average sizes of primary particles can also be at least 10 nm, in particular at least 20 nm and more particularly at least 30 nm. According to particular embodiments, these average sizes can thus be between 10 nm and 100 nm, 20 nm and 80 nm and 30 nm and 60 nm.

As indicated above, these primary particles form aggregates which thus constitute the secondary particles. These secondary particles can more particularly have an average size of at most 150 nm, more particularly of at most 100 nm.

Furthermore, according to another advantageous characteristic of the invention, these secondary particles are themselves also monodisperse. They can in fact have a dispersion index of at most 0.5. This index can be more particularly at most 0.4 and even more particularly at most 0.3.

For the whole of the description concerning the secondary particles, the average size and the dispersion index are the values ​​obtained by implementing the laser diffraction technique using a laser particle size analyzer (mass distribution).

By dispersion index is meant the ratio: σ / m = (d₉₀-d₁₀) / 2d₅Q where:

- d₉₀ is the size or diameter of the particles for which 90% of the particles have a diameter less than d₉₀;

- d₁₀ is the size or diameter of the particles for which 10% of the particles have a diameter less than d₁₀;

- d₅₀ is the average particle size or diameter.

The liquid phase of the suspensions according to the invention can be of different nature.

It can be water first.

It can also be a water / solvent mixture miscible with water. As an example of a solvent of this type, mention may be made of alcohols such as methanol or ethanol, glycols such as ethylene glycol, acetate derivatives of glycols such as ethylene glycol monoacetate or polyols.

The liquid phase can finally consist of an organic solvent.

As an example of an organic solvent, there may be mentioned aliphatic hydrocarbons such as hexane, heptane, octane, nonane, inert cycloaliphatic hydrocarbons such as cyclohexane, cyclopentane, cycloheptane, aromatic hydrocarbons such as benzene. , toluene, ethylbenzene, xylenes, liquid naphthenes. Also suitable are petroleum cuts of the Isopar or Solvesso type (trademarks registered by the company EXXON), in particular Solvesso 100 which essentially contains a mixture of methylethyl- and trimethyl-benzene, Solvesso 150 which contains a mixture of alkylbenzenes in particular of dimethylbenzene and of tetramethylbenzene and Isopar which essentially contains iso- and cyclo-paraffinic C-11 and C-12 hydrocarbons. Mention may also be made, as other petroleum cuts, of those of Petrolink® type from Petrolink or of Isane® type from Total.

It is also possible to use, as organic solvent, chlorinated hydrocarbons such as chloro- or dichloro-benzene, chlorotoluene. Ethers as well as aliphatic and cycloaliphatic ketones such as for example diisopropyl ether, dibutyl ether, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, mesityl oxide, can be considered.

Esters can be used such as those resulting from the reaction of acids with C1 to C8 alcohols and in particular palmitates of secondary alcohols such as isopropanol. Mention may be made of butyl acetate by way of example.

Of course, the liquid phase can be based on a mixture of two or more hydrocarbons or compounds of the type described above.

The suspensions of the invention have an overall content of oxide, that is to say of cerium oxide, which can vary within wide limits and which can, for example, be at most 25% to 30% by mass of oxide. Likewise, the pH of these suspensions can be within a wide range. Thus, the pH of the suspensions resulting from the preparation processes which will be described later is generally between 2 and 5, but this pH can be increased beyond the value of 5 in a known manner, by adding to the suspension of compounds such as anionic or cationic or zwiterionic polymers.

Finally, it will be noted that the suspensions of the invention are stable. This is understood to mean that on these suspensions the formation of a settling cake is not observed for several days, for example at least 8 days. In addition, the settling cake, if it forms, can be resuspended by simple agitation.

Methods of preparing the suspensions of the invention will now be described.

According to a first embodiment, a suspension of the invention can be prepared by a process which comprises the following steps:

- (a) preparing a solution of a cerium III salt which further comprises cerium IV;

- (B) this solution is brought into contact under an inert atmosphere with a base, whereby a precipitate is obtained;

- (c) the medium obtained in the preceding step is subjected to a heat treatment under an inert atmosphere, at least one of steps (a), (b) or (c) being carried out in the presence of nitrate ions;

- (D) is carried out successively but in any order an acidification and washing of the medium thus obtained, whereby the suspension is obtained.

The first step (a) of the above process therefore consists in preparing a starting solution which is a solution of a cerium III salt.

As cerium III salts, use may more particularly be made of cerium III nitrate, chloride, sulphate or carbonate, as well as mixtures of these salts such as nitrate / chloride mixtures.

In a known manner, this starting solution must have the appropriate acidity so that the cerium is indeed entirely present in solution.

The starting solution further comprises cerium IV. Cerium IV is provided by a salt. It can be, for example, cerium IV nitrate.

Generally, the amount of cerium IV is such that the molar ratio (CelV / CellI) in the starting solution is between 1/5000 and 1/50.

The starting solution prepared in step (a) can be degassed beforehand by bubbling an inert gas. By “inert gas” or “inert atmosphere” is meant for the present description an atmosphere or a gas free of oxygen, the gas possibly being, for example, nitrogen or argon.

The second step (b) of the process consists in reacting the starting solution with a base.

As base, it is possible in particular to use products of the hydroxide type. Mention may be made of alkali metal or alkaline earth hydroxides and ammonia. It is also possible to use secondary, tertiary or quaternary amines. However, amines and ammonia may be preferred insofar as they reduce the risks of pollution by alkali or alkaline earth cations.

The base can also be degassed beforehand by bubbling an inert gas.

To carry out the reaction of the second step of the process, the contacting can be done in any order of introduction of the reactants. However, it is preferable to introduce the starting solution into a medium containing the base.

This second step must be carried out under an inert atmosphere, either in a closed reactor or in a semi-closed reactor with sweeping with the inert gas. The contacting is generally carried out in a stirred reactor.

Finally, this second step is generally carried out at room temperature (20 ° C-25 ° C) or at a temperature of at most 50 ° C.

The third step (c) of the process is a heat treatment of the reaction medium obtained at the end of the previous step.

This treatment consists of heating the medium and maintaining it at a temperature which is generally at most 95 ° C and more particularly between 60 ° C and 95 ° C.

The duration of this treatment can be between a few minutes and a few hours.

This treatment is also carried out under an inert atmosphere, which has been described with regard to this atmosphere for the second step which also applies here.

According to one characteristic of the process of the invention, at least one of stages (a), (b) or (c) must be carried out in the presence of nitrate ions. Generally, the nitrate ions are introduced by adding nitric acid, more particularly in step (a), during the preparation of the cerium III solution.

The quantity of nitrate ions, expressed by the NO molar ratio₃'/This³⁺ is generally between 1/3 and 5.

The last step of the process, step (d), in fact comprises two successive operations which can be carried out in any order. These operations are on the one hand an acidification and on the other hand a washing.

These operations will be described more precisely below in the case of a sequence of acidification then washing.

Acidification generally takes place after cooling the medium obtained at the end of step (c) by adding an acid.

Any mineral or organic acid can be used. Nitric acid is used more particularly.

The amount of acid added is such that the pH of the medium after acidification is between 2 and 5.

This operation can be carried out in air; it is no longer necessary to operate under an inert atmosphere at this stage of the process.

The acidification is followed by washing the aim of which is to eliminate the soluble species, essentially salts, from the suspension.

Washing can be done in different ways with or without solid / liquid separation.

It can thus be carried out by separating the solid particles from the liquid phase, for example by front filtration, decantation or centrifugation. The solid obtained is then resuspended in an aqueous phase. It is also possible to proceed by tangential filtration.

This washing can optionally be repeated if necessary, for example until a given conductivity of the suspension is obtained, the conductivity measuring the level of impurities present in this suspension.

As indicated above, the order of the operations can be reversed with respect to what has just been described. Thus, at the end of step (c) and, here too, generally after cooling the medium obtained, it is then possible to carry out washing in the manner described above. At the end of the washing, the acidification of the medium obtained is then carried out.

At the end of step (d), a suspension according to the invention is obtained.

In the case of a suspension partially or totally in an organic solvent medium other than water, this suspension can be prepared in a manner known per se from an aqueous suspension as obtained by the process which comes from be described and by contacting with the organic solvent.

At this stage, it may be advantageous to add to the organic phase a promoter whose function is to accelerate the transfer of the particles from the aqueous phase to the organic phase and to improve the stability of the organic suspensions obtained.

As promoter, it is possible to use compounds containing an alcohol function and very particularly linear or branched aliphatic alcohols having from 6 to 12 carbon atoms. As specific examples, mention may be made of ethyl 2 hexanol, decanol, dodecanol or their mixtures.

The contacting can take place at room temperature, for example around 20 ° C, but also at a higher temperature, for example in a range from 60 ° C to 150 ° C.

The separation between the aqueous and organic phases is carried out for example by distillation, by decantation or centrifugation depending on the nature of the organic solvent.

A second embodiment of the method will now be described.

This second mode differs from the first only by the first step.

This first step consists in preparing a solution of a cerium III salt which also comprises hydrogen peroxide.

What has been described above on the nature of the cerium III salt also applies here.

The amount of solution of H₂O₂ is such that the molar ratio (H₂O₂/ Celll) in the cerium salt solution is between 1/10000 and 1/100.

The remainder of the process according to this second mode takes place as described above for the first mode, that is to say that the solution of the first stage is brought into contact under an inert atmosphere with a base, and a heat treatment is carried out. under an inert atmosphere and the medium thus obtained is acidified and washed (steps (b), (c) and (d) as described above with the presence of nitrate ions in at least one of steps (a) (b) and ( vs)). What has been described previously for all of these subsequent steps and for the first embodiment of the method therefore applies in the same way here for the second embodiment.

The invention also relates to a redispersible powder of cerium oxide particles. This powder has the characteristic that, after introduction into a liquid phase and redispersion in a liquid phase, it results in a suspension according to the invention as described above. The redispersion takes place by simply stirring the powder in the liquid phase.

This powder can be obtained from a suspension according to the invention by drying and then calcining at a temperature which can be for example at most 300 ° C and in particular between 100 ° C and 200 ° C, over a period of time which can vary between a few minutes and a few hours.

The invention also relates to a suspension for polishing comprising either a suspension as described above, or a suspension as obtained by the methods described above, or also a suspension obtained after redispersion of a powder according to the invention. This suspension can be used for polishing glass, for example in the crystal industry, ice cream, flat glass, television screens, glasses, or even for polishing ceramic materials or other materials of the vitreous type. This suspension can also be used quite particularly for CMP type polishing in the electronics industry and therefore for the polishing of metal substrates used in the constitution of microprocessors but also for the polishing of the insulating layers of these same microprocessors, the suspension of the invention being particularly suitable for polishing them. These layers are generally made of silica (doped silica, porous silica).

Generally, such suspensions comprise, in addition to the compound with abrasive property such as the particles of cerium oxide, additives such as a dispersant or an oxidant.

As other applications of the suspensions of the invention, mention may be made of catalysis, in particular for automobile post-combustion, in this case the suspensions are used in the preparation of catalysts. The suspensions can also be used for their anti-UV properties, for example in the preparation of polymer films (of the acrylic or polycarbonate type, for example), paints, papers or cosmetic compositions, in particular in the preparation of anti-UV creams. UV.

Examples will now be given.

EXAMPLE 1

A dilute cerium nitrate solution is prepared by adding 380.8 g of a 2.88 M trivalent cerium nitrate solution, 163.0 mg of a H2O2HOV solution and 877.9 g of water swapped. This solution, with a H2C> 2 / Ce molar ratio of 1/450, is loaded into a semi-closed tank and then degassed with vigorous stirring and bubbling with nitrogen for 2 hours.

A dilute ammonia solution is prepared by adding 839.5 g of deionized water and a solution of 233.2 g of 28% ammonia. This solution is loaded into a semi-closed 2L jacketed reactor and then stirred and bubbled with nitrogen for 2 hours.

The dilute cerium nitrate solution is then added at room temperature to the dilute ammonia solution over 30 min. with stirring and with nitrogen flushing.

The reaction mixture is then raised in temperature to 80 ° C. in% of hours approximately and then maintained at this temperature for 4 h, still under nitrogen purging.

At the end of this heat treatment, the reaction mixture is left to cool and then it is acidified to pH 2 by adding 68% nitric acid. The nitrogen flushing is stopped and the suspension finally washed by centrifugation, elimination of centrifugation water and resuspension of the cake in deionized water. Several washing cycles by centrifugation are carried out until a conductivity of less than 0.5 mS / cm is obtained.

The suspension finally obtained is adjusted to 10% by weight in CeO₂.

The suspension is observed in TEM. It can be seen that the primary particles are indeed monodisperse and of a size centered around 35 nm. On a photo of approximately 150 particles (FIG. 1) representative of the suspension, each of the particles is counted and measured, whereby an average size of 33 nm is obtained with a standard deviation of 12%.

Part of the suspension is dried in an oven at 200 ° C, which makes it possible to obtain a CeO powder₂ for DRX analysis. The X diffractogram of this powder has the signature of CeO₂ crystallized (ASTM file 34-394). The average size of the coherence zone calculated from the width at mid-height of the diffraction peaks located at 2Θ = 28.6 °, 47.5 ° and 56.4 ° by application of the Scherrer model gives 40 nm. The BET specific surface determined by nitrogen adsorption is 23.5 m²/ g, which gives an average primary particle size of 35nm.

The size of the secondary particles is measured using a laser particle size analyzer of the Horiba LA910 type by considering a value of the optical index of CeO₂ in water of 1.7. The median size d₅o is 102 nm. The dispersion index σ / m calculated from the values ​​d₁₀, dso and d₉₀respectively 81, 102 and 128 nm is 0.23.

EXAMPLE 2

A dilute cerium nitrate solution is prepared by adding 6.4 kg of a 2.88 M trivalent cerium nitrate solution (d = 1.715), 1.0 kg of a HNO solution₃ 68%, 4.8 kg of deionized water and 8.87 g of 1.39 M tetravalent cerium nitrate (d = 1.440). This solution, of molar ratio

This⁴7This_totai of 1/1250, is loaded into a semi-closed tank and then degassed with vigorous stirring and bubbling with nitrogen for 2 hours.

A dilute ammonia solution is prepared by adding 22.6 kg of deionized water and a 4.6 kg solution of 28% ammonia. This solution is loaded into a semi-closed 40L jacketed reactor and then stirred and bubbled with nitrogen for 2 hours.

The dilute cerium nitrate solution is then added at room temperature to the dilute ammonia solution over 30 min. with stirring and with nitrogen flushing.

The reaction mixture is then raised in temperature to 83 ° C. in% of hours approximately and then maintained at this temperature for 4 h, still under nitrogen purging.

At the end of this heat treatment, the reaction mixture is left to cool and then it is acidified to pH 2 by adding 68% nitric acid. The nitrogen flushing is stopped and the suspension finally washed by centrifugation, elimination of centrifugation water and resuspension of the cake in deionized water. Several cycles of washing by centrifugation are carried out until a suspension finally adjusted to 10% by weight of CeO2 having a pH of 3.2 and an ionic conductivity of 0.38 mS / cm is obtained.

The suspension is observed in TEM. It is observed that the primary particles are indeed monodisperse and of size centered around 60 nm. On a photo of several hundred particles (FIG. 2) representative of the suspension, each of the particles is counted and measured, whereby an average size of 65 nm is obtained with a standard deviation of 9%.

Part of the suspension is dried in an oven at 200 ° C., which makes it possible to obtain a CeO2 powder for the XRD analysis. The X diffractogram of this powder has the signature of CeO₂ crystallized (ASTM file 34-394). The average size of the coherence zone calculated from the width at half height of the diffraction peaks located at 20 = 28.6 °, 47.5 ° and 56.4 ° by application of the Scherrer model gives 70 nm. The BET specific surface determined by nitrogen adsorption is 13.0 m²/ g, which gives an average primary particle size of 64 nm.

The size of the secondary particles is measured using a laser particle size analyzer of the Horiba LA910 type, considering a value of the optical index of CeC> 2 in water of 1.7. The median size d₅o is 112 nm. The dispersion index σ / m calculated from the dio, dso and dgo values ​​of 84, 112 and 157nm respectively is 0.32.

EXAMPLE 3

A suspension of cerium oxide is prepared according to the same process as in Example 2 but with a molar ratio Ce⁴7This_totai in the diluted cerium nitrate solution of 1/1000 instead of 1/1250.

The suspension is characterized by a concentration of 10% by weight in

CeO₂ having a pH of 3.5 and an ionic conductivity of 0.18 mS / cm. The average size of the coherence zone calculated from the width at mid-height of the diffraction peaks located at 20 = 28.6 °, 47.5 ° and 56.4 ° by application of the Scherrer model gives 54 nm. The BET specific surface determined by nitrogen adsorption is 19.0 m²/ g, which gives an average primary particle size of 44 nm. The size of the secondary particles is measured using a laser particle size analyzer of the Horiba LA910 type by considering a value of the optical index of CeO₂ in water of 1.7. The median size d 50 is 110 nm. The dispersion index σ / m calculated from the values ​​dw, dso and dgo respectively of 85, 110 and 147 nm is 0.28.

The suspension is dried for 1 night at 100 ° C. then 4 hours at 200 ° C., which makes it possible to obtain a powder.

This powder is redispersed in deionized water. The suspension thus obtained is characterized by a concentration of 10% by weight of CeO₂, a pH of 3.5 and an ionic conductivity of 0.21 mS / cm. The size of the secondary particles is measured using a laser particle size analyzer of the Horiba LA910 type by considering a value of the optical index of CeO₂ in water of 1.7. The median size d₅₀ is 93 nm. The dispersion index σ / m calculated from the values ​​dw, d₅₀ and D₉₀ 76, 93 and 116 nm respectively is

0,21.